In the gasification of a hydrocarbon fuel such as coal or coke, for example, the fuel, in particulate form, is fed into the gasifier reaction chamber together with an oxidizing gas. Reaction of the particulate fuel with the oxidizing gas results in the production of a raw synthesis gas which is carried from the gasifier for further treatment. The events within the reaction chamber produce not only a usable gas, but also a slag having a constituency which depends to a large degree on the fuel being burned. Since the gasifier for this purpose must be operated at a relatively high temperature and pressure which is well known in the industry, conditions within the combustion chamber must be monitored at all times.
Normally, gasifiers are equipped with one or more temperature monitoring devices. One such device is the thermocouple, a plurality of which may be disposed throughout the refractory lined walls of the gasifier reaction chamber. The thermocouples are placed in the gasifier in such a way that they are separated by a thin layer of refractory from the flames in the reaction chamber. This is done to protect the relatively fragile thermocouple junctions from the environment inside the reaction chamber. Consequently, the thermocouples do not sense the reaction temperature directly, but instead respond to the heat transmitted through the refractory layer of the reaction chamber. It should be appreciated that, as a result of the lagtime inherent in conductive heat transfer, there may be a substantial delay in thermocouple response to changes in temperature within the reactor. This is especially true during gasifier startup when reaction initiation results in a rapid temperature rise which must be detected in order to confirm that the reactions have initiated. In addition, heat transfer lagtimes effect thermocouple response to operating condition changes during normal gasifier operation.
As an alternative to thermocouples, pyrometers are sometimes used to measure reaction temperature. Physically, the pyrometer is mounted external to the reactor and views the reaction chamber through a gas purged sight tube which normally extends from the pyrometer into the reaction chamber.
A major weakness of the pyrometer temperature monitor arises from the difficulty encountered in keeping the sight tube free of obstructions. The potential for obstruction is great, resulting from the atmosphere within the reaction chamber which is characterized by rapid swirling of particulate carrying gas. Further, a slag which results from ungasifiable material within the fuel, will likewise swirl around the reaction chamber, contacting the walls of the latter. In the course of gravitating towards the lower end of the gasifier, slag normally displays a tendency to cling to the reaction chamber walls. The clinging slag and the swirling particles interfere with the operation of the pyrometer sight tubes which are positioned in the reaction chamber walls. In addition, during the gasifier startup sequence, fuel is introduced into the reactor before oxidant. Depending upon the circumstances and upon the fuel, coal-water slurry for example, there exists an increased tendency for obstruction of the pyrometer sight tubes with unreacted fuel.
Alternately, if a nonprocess gas (e.g. an inert gas such as nitrogen) is used as the purge gas, the product from the reaction chamber will be slightly diluted by the pyrometer purge gas. If the gasifier is producing a synthesis gas for a chemical process, the presence of a diluent gas may not be acceptable.
In order to maintain a clear line of sight into a high pressure reactor as well as the pressure integrity of the reactor vessel, optical access ports involving elaborate high pressure sight glasses are required. For example, in the case of coal gasification, a gas purged sight glass arrangement (for example, see U.S. Pat. No. 5,000,580) is used to keep the sight glass clear of the molten slag and solid particles which swirl around the inside of the gasifier. For safety reasons, a shutoff valve connected to an emergency shutdown system is also used to prevent the gasifier from depressuring through the optical access port in the event that a sight glass breaks.
Current optical access ports are effective and reliable. However, they are expensive, they introduce an added safety concern into the process (because of the slight potential for sight glass breakage) and the required purge gas is sometimes an unwanted diluent in the reactor product. Also, because of their size, they make it difficult to obtain more than one process measurement through a single optical access. Modern process safety systems, however, often require triply redundant measurements.
Under certain circumstances, it is possible to eliminate the complicated high pressure sight glass and optical pathway purge system by using elements of the gasification process itself For example, in natural gas gasification, where a two-stream process injector is used, the oxygen lance feed tube provides a completely unobstructed sight path of circular cross-section into the reaction chamber. The flowing oxygen itself serves as the purge gas. And, because the reaction zone at the exit of the process injector is entirely gaseous, there is nothing (no solid or liquid particles) to obstruct the optical pathway into the interior of the reaction chamber. U.S. Pat. No. 5,281,243 shows one such scheme for measuring gasifier temperature through the process feed injector oxygen lance.
Even though the inventions in U.S. Pat. Nos. 5,000,580, and 5,281,243 are successful, the present invention greatly improves the ability to measure gasifier temperature by simplifying the optical access by the elimination for the need of a optical site window. In addition, the present invention makes the measurement system more rugged and durable, given the harsh conditions of the reactor, while also allowing one to obtain triply redundant temperature measurements which previously has not been possible.